1. Field of the Invention
This invention relates to methods for packaging hot melt adhesives, especially hot melt adhesives which exhibit tack at room temperature. The method involves the continuous enrobing of the adhesive in a non-tacky casing at low pressure.
2. Description of the Prior Art
Hot melt adhesives are compositions that are solid at ambient room temperatures. The adhesives are melted at their application temperature, ranging from usually 110.degree. C. to 180.degree. C. (230.degree. F. to 350.degree. F.) at which temperature they are fluid. Those hot melts which have no tack under the conditions of packaging or storage do not require special techniques for packaging unless they are to be shipped or stored in hot climates. They are usually sold in the form of chicklets, pellets, bars etc., and packaged into boxes, bulk bags, etc. The hot melt adhesives which are pressure sensitive or exhibit tack under the conditions of packaging and storage, require special handling. These adhesives consist of compositions that have lower softening points and can stick to each other in storage. They also show cold flow properties under conditions of packaging and storage. Several attempts have been made to package these materials so as to avoid these problems. These adhesives have been supplied in special drums such as those supplied by Grief Brothers, and are lined with silicone-coated surface. The customer using these drums, however, requires either a drum melter or a drum unloader, both of which are very expensive. This method of packaging and storage is only useful and economical for those customers who have a high rate of consumption of adhesives.
Another method used to package these adhesives is in silicone-coated cardboard boxes such as those supplied by Menasha Corporation. These boxes are available in different sizes but are expensive and add substantial cost to the adhesive. Moreover, the filling and handling of these boxes is labor-intensive as well.
A third method commonly used to package these adhesives is in multi-compartment trays. In this method, the multi-compartment (multiple cavity) trays made of plastic or metal are first coated with a powdered wax polymer or copolymer or sprayed with molten wax. The molten adhesives is then poured into the trays while the trays are floating in chilled water or other heat sink. The adhesive is allowed to solidify as the powdered wax fuses with the surface of the block. The newly formed blocks of adhesive are then removed form the cavities in the trays. The unwaxed top surface of the blocks are then either coated with wax or the uncoated surface of the blocks are joined together to make a double-block of adhesive. The block or double-block now has wax on all sides and a non-tacky surface (U.S. Pat. Nos. 4,748,796 and 4,755,245). The wax powder used is so selected that it gives a non-tacky surface and it becomes part of the adhesive when the blocks are melted in the glue pot.
This method has several drawbacks. First, it involves a separate process to coat the trays with powder. Second, there is usually excess powder in the trays which does not stick to the molten adhesive. On cooling, when the adhesive block is removed from the trays, the excess powder falls on the floor creating a slippery and hazardous floor. Third, when the customer handles the blocks, he too experiences the loss of excess powder on his floors, again creating hazardous conditions. Fourth, the powder does not provide any strength to the block of adhesive. When these blocks are packaged into boxes, the blocks of adhesive flatten during shipments in the summer months due to cold flow. U.S. Pat. No. 5,725,820 discloses an improvement wherein a non-tacky polymer is used in place of a powder.
Another method of packaging the hot melt adhesive is co-extrusion. With this method, the non-tacky component of the adhesive is co-extruded as outside surface, and the co-extruded adhesive is pinched off to form pieces of adhesive shaped as pillows. This method works well for making very small sized blocks continuously, but the equipment for the co-extrusion is very expensive. The method also is not practical for making large blocks weighing 1.0 to 2.5 kg (2 to 5 pounds) each, which is the commercially preferred size for many applications.
U.S. Pat. No. 5,257,491 issued to Rouyer, et al. discloses wrapping the solid portions of hot melt material in a plastic packaging film in the form of a wrap, sac or bag. This method is an improvement over prior techniques to make hot melt in block form. However, this method involves two steps, i.e., making solid blocks of adhesive and then wrapping or enclosing them in plastic film.
U.S. Pat. No. 5,333,439 issued to Bozick, et al. discloses a method of wrapping hot melt as a continuous process using a continuous sheet of heat sealable film, the film being patterned with a silicone coating. This method also discloses packaging the adhesive in tubular form, using commercially available chub packaging machinery from the KartridgePak company in Davenport, Iowa.
U.S. Pat. No. 5,373,682 issued to Hatfield, et al. discloses a method of continuous packaging that involves pouring molten hot melt into a cylindrical plastic tube. The patent also discloses the use of commercially available chub packaging machinery from the KartridgePak Company. The chub packaging machine, as disclosed in U.S. Pat. No. 5,373,682, consists of a vertical mandrel lined with a plastic tube. The process involves filling the plastic tube with adhesive, pinching the filled plastic tube, and then cutting the plastic tube where it is pinched. Two disadvantages with this method are realized. One, if the adhesive breaks through the film due to a faulty film, if there is poor sealing when the plastic tube is formed, or if there is a break in the film, the tacky adhesive will get on the sides of the mandrel and drop into the voider rolls. This involves a lot of down-time to clean the adhesive from the tube and voider rolls. Second, any air trapped into the cylinder is difficult to remove, especially with high-viscosity adhesives. Entrapped air causes splattering of hot adhesive when the cylinder is added to a pot of molten adhesive.